Synthesis of transition metal doped lanthanum silicate oxyapatites by a facile co-precipitation method and their evaluation as solid oxide fuel cell electrolytes

Transition metal doped apatite La10Si6−xCoxO27−δ (x = 0.0; 0.2; 0.8) and La10Si5.2Co0.4Ni0.4O27−δ are synthesized by co-precipitation method followed by sintering. The precursor precipitates and apatite products are characterized by XRD, FTIR, TGA/DTA, Raman Spectroscopy, SEM-EDX and electrochemical impedance spectroscopy. The presence of apatite phase with hexagonal structure is confirmed through the XRD results. The conductivity measurements of the samples sintered at 1000 °C show that the ionic conductivity increases with increasing content of Co2+ doping into apatite that is further increased by co-doping of Ni2+. The Co doped apatite (La10Si5.2Co0.8O27−δ) exhibited conductivity of 1.46 × 10−3 S cm−1 while Co–Ni co-doped sample (La10Si5.2Co0.4Ni0.4O27−δ) exhibited highest conductivity of 1.48 × 10−3 S cm−1. The maximum power density achieved is also for Co, Ni co-doped sample i.e., 0.65 W cm−2 at 600 °C. The results represented show that Co and Ni enhances the SOFC performance of apatite and makes it potential electrolyte candidate for solid oxide fuel cell application.


Introduction
The advancement of industrial technology and rapid growth of world population has resulted in a high demand for energy. The increase in the demand for energy has generated a great interest to develop the devices which will provide cheap, renewable energy. One of such devices is the solid oxide fuel cells (SOFCs), an electrochemical device that is an efficient source of electrical energy to comply with the future energy demand [1][2][3] . The SOFC is made up of the electrodes, electrolyte, and interconnect materials all in the solid state. The electrolyte materials for SOFCs must have a wide range of characteristics including negligible electronic conductivity, high ionic conductivity (oxide ion or proton), high ion transport numbers in a wide range of oxygen partial pressures, and chemical stability at high temperatures under partially reducing/oxidizing conditions. 4 The stabilized zirconia, which has the uorite structure, was rst used in SOFC in 1937, although it had previously been employed as the electrolyte in the Nerst-lamp 5 . The most widely used stabilizers are yttrium (YSZ) or scandium (SSZ). 6 The conductivities of these solid solutions increase with the degree of substitution to an optimum 8% for Y 2 O 3 and 11% for Sc 2 O 3 , 7 and this maximum conductivity is reached at a degree of substitution close to the minimum that stabilizes the cubic uorite phase. 8 The scandium stabilized zirconia (SSZ) experiences higher ionic conductivities than Yttrium stabilized zirconia (YSZ) but the SSZ is relatively much expensive. 9 Consequently, YSZ has become the electrolyte most widely used in SOFCs that shows good properties as an electrolyte material. However, the major drawback is its high working temperatures (800-1000°C) depending on the thickness of the electrolyte that is required to achieve sufficient ionic conductivity. This high temperature requirement limits the use of other cell components to have high temperature resistance in order to be compatible with the electrolyte material and also high temperature sealing must be used, which causes additional cost increase of the SOFC system. One of the focus areas of SOFCs research is the development of the electrolyte material that would operate at relatively lower temperatures. In this research, we exploit the lanthanum silicate apatites as electrolyte material for SOFCs. Nakayama et. al., were the rst to discover oxyapatite-structured materials with the general formula RE 9.33+x Si 6 O 26+1.5x (where RE represents rare earth elements) 10 . Since then, they have attracted particular attention because of their high oxide ion conductivity and low activation energies. An interesting aspect in materials science is the modication of properties of materials through doping. This is the approach utilized in this research to seek superior ionic conductivity of lanthanum silicate oxyapatites. These studies showed that nonstoichiometry in the form of either cationic vacancies or excess oxygen is required to achieve good oxide ion conductivities 11,12 . Different types of doping on the lanthanum site or on the silicon site, or on the both have been studied. Some studies of the Co-doped lanthanum silicate oxyapatite systems, La 10 -Si 6−x Co x O 27−x/2 , synthesized by the sol-gel method have been reported by Qingle et. al. 13 The results showed that as the Co level increased, an initial increase in conductivity was observed, reaching a maximum value for x = 0.8 (La 10 Si 5.2 Co 0.8 O 26.6 ). Higher Co 3+ concentrations (0.8 < x < 1.5) results in a decrease in the conductivity of La 10 Si 6−x Co x O 27−x/2 because of the lower concentration of interstitial oxide ions and greater stoichiometry. Therefore, excess Co 3+ dopant is unfavorable for high oxide conductivity. The doping results as shown above indicate clearly that oxygen over-stoichiometry is responsible for the good oxide-ion conductivity.
The level and doping site both are important to improve the conductivity. In general, it has been shown that the ionic conductivity decreases when doping is carried out at the lanthanum site, such that the conductivity of La 8.67 SrSi 6 O 26 is found to be 8.3 × 10 −5 S cm −1 at 500°C that is lower than 1.1 × 10 −4 S cm −1 for La 9.33 Si 6 O 26 (ref. 14) because of decrease in the number of cationic vacancies.
Different synthesis methods have been reported for the nanocrystalline lanthanum silicate powders, distinguished according to preparation temperature as solid state reaction and solution synthesis. 15 The high preparation temperature methods have the common problem that precursors do not properly mix on a large scale.
Low-temperature preparation methods have greatly overcome the problem of precursor contact; however, side products are still detected in the compounds obtained with these methods. Recently, some authors have 16,22 reported a facile coprecipitation method to synthesize nano-sized LSO powders, where La(NO 3 ) 3 $6H 2 O and tetraethyl orthosilicate (TEOS) are dissolved in water and ethanol to precipitate well-mixed precursors under dilute ammonia solution. Qingle et. al. 13 used the sol-gel method for the synthesis of La 10 Si 6−x Co x O 27−x/ 2 , with starting Co 3+ ions. While the present work reports the modied co-precipitation method based on a single source metal-organic precursor, that is, the metal octanoate to synthesize ne powders of apatite La 10 Co x Si 6−x O 27−d , where x is maintained as 0.0, 0.2 and 0.8. In addition to doping with Co 2+ , co-doping with Co and Ni in the sample (La 10 Si 5.2 Co 0.4 Ni 0.4 -O 27−d ) is also synthesized in this work, which have not yet been reported. The synthesized samples were calcined at different temperatures and characterized by TG/DTA, XRD, FT-IR, Raman spectra and SEM-EDX to identify the pure phase formation and the morphologies. The effect of partial substitution of silicon by divalent cobalt, Co 2+ and Ni 2+ on the structure and grain morphology are examined. Electrical conductivity properties as a function of temperature under air by electrochemical impedance spectroscopy are investigated to determine the application of material as an electrolyte for SOFC. 2 O of analytical grade were purchased from Sigma-Aldrich. The tetraethyl orthosilane (98%, Acros Organics), octanoic acid (99.99%, Merck), sodium hydroxide (98%, BIOCHEM Chemopharma United Kingdom), absolute ethanol (Baker) were used as received without further purication. De-ionized (DI) water was used throughout the study.

Synthesis of apatites
The cobalt-doped lanthanum silicate of the general formula La 10 Si 6−x Co x O 27−d (where x = 0.0; 0.2; and 0.8) were synthesized in this work. For each composition, the required stoichiometric amounts of lanthanum nitrate, cobalt(II) nitrate, nickel nitrate and tetraethyl orthosilicate (TEOS) were dissolved in DI water along with continuous stirring to obtain a homogeneous solution. The sodium octanoate was used as precipitating agent that was prepared by mixing the solutions of octanoic acid and sodium hydroxide. The solutions containing the metal salts (La 3+ , Co 2+ , Ni 2+ and Si 4+ ) and sodium octanoate were mixed with continuous stirring for about 1 hour at room temperature. The resulting precipitates of precursor (La-Si-Co or La-Si-Co-Ni octanoate) were then ltered to remove the NaNO 3 , followed by washing with ethanol and drying in an oven for 12 hours at 80°C. The as-prepared precursor powder was sintered for 4 h in air at different temperatures in a tubular furnace in air (heating rate 10°C min −1 ).
The cobalt-doped lanthanum silicate of the general formula La 10 Si 6−x Co x O 27−d with x = 0.0; 0.2 and 0.8 are assigned the codes LSCO-0, LSCO-2 and LSCO-8 respectively, while the various precursors from which they are derived are labelled with the codes P-00, P-02, P-08. The compound La 10 Si 5.2 Co 0.4 Ni 0.4 -O 27−d is assigned the code LSCNO and its precursor PCN-04.

Characterizations
In order to determine the calcination temperature, the precursors were analysed by thermogravimetric analysis (TGA). The TGA measurements were performed using DTA 1600 TA Instruments, USA, under oxygen ow rate of 100 ml min −1 , up to a temperature of 900°C, with a heating rate of 20°C min −1 . The crystalline phases were identied by X-ray diffraction (XRD) analysis (Bruker AXS D8 Focus) using Co Ka radiation (l = 1.78901 Å). The angular scan range is 20°-100°and the scan increment is 0.02°per step for all samples. The microstructure characterization of the obtained powders is carried out by scanning electron microscope (JSM5800LV, JEOL, Japan) combined with an EDX holder. The FTIR spectra of samples were recorded using IR Fourier spectrometer Nicolet 6700 equipped with Smart Orbit™ diamond ATR accessory (Thermo Scientic, USA). The FTIR Spectra were obtained in the range of 400-4000 cm −1 . Electrical conductivity measurements as a function of temperature were performed by the complex impedance spectroscopy technique, using a Solartron 1260A frequency response analyser (FRA) over the frequency range 5 Hz to 15 MHz, under static air. The measurements were made in open circuit and with an applied AC voltage of 100 mV. Impedance diagrams are recorded from 450 to 750°C with thermal steps of about 25°C and settling times of 20 minutes. The fuel cell performance were performed with hydrogen fuel. The measurments were performed at 600°C.

Results and discussion
3.1. Characterization of precursors 3.1.1. Analysis by Fourier transform infrared spectroscopy (FTIR). The FTIR spectra of the octanoate precursors of LSCO-0, LSCO-2 and LSCO-8 samples are presented in Fig. 1. It can be observed that all the spectra are similar, i.e. that all the peaks emerge at practically the same positions and therefore representing the presence of same functional group. These FTIR spectra allow us to deduce that cobalt doping has no inuence on the functional groups in the samples, which is normal because the addition of cobalt does not modify the chemical nature of the existing ligands in the precursor compound.
The absence of vibration of the free carbonyl (C]O) around 1720 cm −1 and 1740 cm −1 is an indication that the carboxylate group of the octanoate has fully engaged in the coordination bond. From Fig. 1, the intense peaks at 2925 and 2840 cm −1 are typical of symmetric and asymmetric stretching vibrations of C-H in the -CH 3 and -CH 2 groups respectively. 17 The peaks at 1418 cm −1 and 1200 cm −1 are attributed to the scissors deformation and wagging vibrations of C-H, respectively. Although the characteristic band of the C]O group of carbonyl compounds and carboxylic acids is missing due to the coordination bond formation but the two CO groups in the octanoate ligands give rise to asymmetric and symmetric frequencies at 1601 and 1526 cm −1 , respectively. 17 This is evidence of the presence of carboxylate groups in the samples. It can be demonstrated that the value of the separation between the asymmetric and symmetric stretching of COO, i.e., Dn (yCOOasy -yCOOsym) can be used to determine the binding mode between the carboxylate group (COO) of the ligand and the metal. The value of Dn below 140 cm −1 is attributable to the bidentate chelating mode while Dn between 140 cm −1 and 200 cm −1 is attributed to the bridged bidentate binding mode and Dn above 200 cm −1 is attributed to monodentate binding mode. 18 From the spectra, it is observed that the octanoate ligands exhibit bidentate chelating effect to the metal in all the compounds. These results are similar to those reported in the literature for similar compounds. 19 These results conrm the presence of the expected functional groups in the precursors: P-00, P-02, and P-08.
3.1.2. Thermal behavior and phase formation of the La 10 -Si 5.8 Co 0.2 O 27−d precursor. Thermogravimetric analysis (TGA) of the precursor of the La 10 Si 5.8 Co 0.2 O 27−d sample was performed to determine its decomposition temperature as plotted in Fig. 2. The differential thermal analysis curve showed that there are three major regions of weight loss. The rst region falls around 100°C that is due to removal of free and some of bound water. The second region falls between 350 and 450°C that is attributed to the decomposition of organic part present in the sample. This temperature range is responsible for decomposition of single metal octanoates. 20 There is a small decomposition around 700°C. The total weight loss from 100°C and 700°C is about 73% that is mainly corresponds to the loss of the organic fraction (theoretical percentage 76%). However, there is no further weight loss observed at temperature above 800°C, which conrms the absence of any further decomposition. The residue le aer complete decomposition was 26.91% that is quite close to the theoretical value of 23.54%, which corresponds to the mixed oxide La 10 Si 6−x Co x O 27−d . From the TGA  curve, it is considered that 800°C is a suitable temperature to treat the precursor for the synthesis of apatites.  Fig. 3. The XRD patterns are indexed on the standard ICCD card No. 053-0291. All the experimental peaks matched with the standard indicating oxy-apatite phase. The synthesized samples thus exhibit hexagonal structures with space group P6 3 /m. La 2 O 3 peaks were absent. Some other peaks marked with an asterisk (*) are observed that might correspond to the apatite phase La 10−x Si 6 O 26+d , where 0 < x < 1 since such peaks appear in the standard pattern of La 9.31 Si 6.24 O 26 and/or NO 3 − (ICDD card No 40-0234). Other researchers obtained the apatite-type lanthanum silicates from various synthesis methods including co-precipitation, sol-gel and also reported hexagonal structure. 13,22 However, we observed that the apatite phase was formed at temperatures from 800°C, which is much lower than the other methods described in the literature. Lattice parameters and unit cell volume for the hexagonal structure were calculated from the X-ray diffraction data and are presented in Table 1 along with some values from the literature. It is observed that there is a general increase in the unit cell volume and lattice parameters with increasing dopant content.  The particle size was calculated from the line broadening of the most intense peak using the Scherer formula (D = 0.9l/b cos q), where D is the particle size, l is the wavelength of the radiation, ß is the full width at half maximum, FWHM and q is the diffraction angle. All the particles are observed to fall in the nanometer range.
3.2.3. Raman spectroscopy. The structure of apatite is composed of a quasi-compact arrangement of tetrahedral SiO 4 groups. As the Si-O bonds in the tetrahedra are stronger than the La-O bonds, 23 the vibrational spectrum of the apatite will be divided into internal vibrations due to strong bonds in the SiO 4 , and external vibrations that is linked to the rest of the structure. The internal vibration modes of the isolated SiO 4 unit are divided into four vibration modes including a symmetrical elongation mode of frequency y 1 , a symmetrical mode of angular deformation y 2 , an asymmetric mode of elongation y 3 , and an asymmetric mode of angular deformation y 4 . Vibration modes y 1 and y 2 are more intense in Raman spectroscopy. 23,24    asymmetric stretching modes (n 3 ) of the tetrahedral SiO 4 unit molecule. In addition, another such intense band at 842.92 cm −1 is attributed to the symmetric bending mode (n 1 ) and the band at 453.24 and 520.95 cm −1 is attributed to the asymmetric bending mode (n 4 ) of the SiO 4 tetrahedral unit. The Raman band observed at 282.72 cm −1 , and all other bands below it correspond to the La-O (SiO4) vibration.
3.2.4. Microstructure and elemental analysis of prepared samples. In order to understand the density, grain boundaries and especially the phase purity, the SEM technique is used. 25 Fig. 7 shows the Nyquist plot of the impedance spectroscopy performed on LSCO and LSCNO samples at different temperatures (450°C, 550°C, 650°C and 750°C). At higher temperature like 750°C, the spectrum of the compound clearly shows two semicircles with different frequencies corresponding to the bulk resistance (R b ), and the grain boundary resistance (R gb ) respectively. 13,26 At low temperatures (450°C and 550°C), the spectra are composed of two semicircles for LSCO-0 and LSCO-2 which may correspond to the electrode resistance, R el and the material's resistance. The bulk resistance, R b and the grain boundary resistacne, R gb are indistinguishable. 13,26 For LSCO-8 and LSCNO at low temperature, only one semicircle is observed for the electrode resistance. As the temperature increases, all the samples show two semicircles with different frequencies corresponding to the electrode resistance, R el and materials resistance (bulk resistance (R b ) and grain boundary resistance, R gb ). The increase in temperature results in decrease of the semicircles.
The total oxygen ion conductivity of the samples operating at different temperatures was calculated using the following equation: where L is the thickness (cm) of the pellet, A is its surface area (cm 2 ), and R is the total resistance (U) of the sample (grains and grain boundaries). Fig. 8(a) reports the total conductivity of the different compositions carried out in this work at temperatures ranging from 450 to 750°C. From Fig. 8(a), it is observed that the ionic conductivity gradually increases with temperature and reaches to its maximum value at 750°C. This is evidence that the ionic diffusion process is a dominant process that is activated by heat. 27 Fig. 8(b) shows the plot of the logarithm s(T) versus 1000/T of the calculated electrical conductivity (s) for different samples sintered at 1000°C during 4 hours and the data points are tted to the Arrhenius equation: where s is the ionic conductivity (S cm −1 ) of the sample, T is the temperature (K), E a is the activation energy (eV), k is the Boltzmann's constant (8.62 × 10 −5 eV K −1 ), and s 0 is the preexponential term of Arrhenius' law. The line tting indicates that the oxide ion diffusion process is thermally activated. 28 The Arrhenius diagram in Fig. 8(b) shows us a linear evolution of the ionic conductivity. The linear evolution of ionic conductivity indicate that the conductivity is thermally activated, and the activation energy is 0.30, 0.28, 0.39, 0.40 eV for sample LSCO-0, LSCO-2, LSCO-8, and LSCNO, respectively. The values are relatively less in comparison with the literature data (0.5-0.9 eV without impurity phase). 29 It is also observed that the effect of Co 2+ doping increases the conductivity of the different materials. This effect is shown in Fig. 8(c). The best value obtained in the case of our work is for x = 0.8 at 750°C with a value of 1.48 × 10 −3 S cm −1 . These results are similar to those obtained by Qingle et al. 13 who also showed that the conductivity increased with the doping level of cobalt up to a certain threshold and then decreased beyond x = 0.8. The sample that was co-doped with nickel and cobalt, La 10 -Si 5.2 Co 0.4 Ni 0.4 O 27−d has conductivities at temperatures 550°C and 750°C very slightly higher than the x = 0.8 sample. However, the conductivity of these sample increases exponentially with temperature increase from 550°C to 650°C and then even higher conductivity values achieved at 750°C. Ni doped lanthanum silicate oxyapatite, La 9.33 Si 6−x Ni x O 26−x (x = 1.0) showed conductivity of 1.21 × 10 −3 S cm −1 at 700°C 30 while the Co and Ni co-doped sample, La 10 Si 5.2 Co 0.4 Ni 0.4 O 27−d in this work showed a higher value of 1.48 × 10 −3 S cm −1 at 750°C.
The La 10 (SiO 4 ) 6 O 3 compound is made up of isolated SiO 4 units forming two distinct types of interstices parallel to the caxis. The smaller insterstice contains La 3+ cations while the larger one is occupied by La 3+ and O 2− ions. 31 Conduction is through the interstitial oxide ions. Co doping at the Si sites creates cation vacancies. The oxide ions can hop through these vacant sites, 14 which results in increase in the ionic conductivity as shown with higher Co doping content.
3.2.6. Fuel cell performance with hydrogen. All the prepared electrolyte samples were used to prepared the fuel cells to evaluate their performance as shown in Fig. 9. The testing unit LC-43 from China is used for I/V measurements at 600°C. The three apatite samples with and without Co doping; LSCO-O, LSCO-2, LSCO-8 showed the fuel cell performance as 0.290 W cm −2 , 0.230 W cm −2 and 0.130 W cm −2 , respectively,